Across a swath of northern Nigeria, a humanitarian catastrophe is unfolding, as lead from illegal gold mines sickens thousands of children.

More than 400 kids have died, and many more have been mentally stunted for life.

Doctors Without Borders, which has set up clinics to treat the children, is calling it one of the worst cases of environmental lead poisoning in recent history.

The problem first came to light in 2010, when children in some villages started dying. Medical professionals couldn't even explain what was happening. In some villages, one-third of children under 5 years old died.

"Initially, they and we thought there was some sort of communicable disease. It felt like a hemorrhagic fever or something," says Ivan Gayton, who heads the mission for Doctors Without Borders in Nigeria.

Gado Labbo, a mother in the tiny village of Dareta, says she had no idea what was wrong with her 3-year-old son, Yusuf, a few years ago. He was a healthy child, and then suddenly he started violently convulsing.

When Yusuf first entered a Doctors Without Borders clinic in 2010, the level of lead in his blood was 150 micrograms per deciliter — 30 times the level considered dangerous by the U.S. Centers for Disease Control and Prevention.

Now Yusuf is 5 years old. He's blind, weighs just 22 pounds, can't speak or walk and spends most of his days clutched in his mother's arms.

Clinicians have brought his blood level down to about 50 micrograms per deciliter, says Susan Lake, a nurse from Doctors Without Borders. But he has already suffered significant neurological damage, and the long-term prospects for Yusuf are not good.

These problems are "clearly linked to the lead," Lake says. "Obviously, with children, we just have to wait and see, but there isn't much hope that he would gain enough capacity to walk and talk again."

Unfortunately, the same is true for other children who have been exposed to high levels of the metal.

Women and their children wait for medication at the clinic in Dareta, Nigeria. Treating children with high levels of lead is a painstaking process that works only if their environment at home is free from lead.

Ivan Gayton directs the mission for Doctors Without Borders in Nigeria. The nonprofit set up clinics to treat children sickened by lead poisoning from illegal mining activities. Gayton says this may be one of the worst cases of environmental lead poisoning in recent history.

Gado Labbo holds her 5-year-old son, Yusuf, at the clinic in Dareta. In 2010, when Yusuf first entered the clinic, he had a blood lead level of 150 micrograms per deciliter — 30 times higher than what the Centers for Disease Control and Prevention considers dangerous.

A health worker looks for signs of vision in the eyes of Yusuf Labbo. Severe lead poisoning has left the 5-year-old boy blind. He weighs just 22 pounds, can't speak or walk and spends most of his days clutched in his mother's arms.

Susan Lake, a nurse for Doctors Without Borders, goes over the list of pills for one of her patients at the clinic in Dareta. More than 400 kids have already died from lead poisoning, while thousands of others are stunted physically and mentally.

A local health official stands in the consultation room of a health clinic and hospital in Bagega, Nigeria. At first, miners in the region were processing lead-laden gold ore inside their homes. Doctors Without Borders persuaded most of them to move the processing to the outskirts of town.

The source of the lead is illegal gold mines, also called artisanal mines, which have risen in popularity as the price of gold has gone up in the past few years.

Gold is mingled with veins of lead in this part of Nigeria, and miners use primitive methods to process the raw ore. This puts large amounts of lead-laden dust into the air.

Just outside the village of Bagega, men smash piles of rocks with salvaged auto parts. The smaller bits of rock are then crushed further in motorized flour mills.

Dust flies up in the men's faces and coats their clothes.

The sand that comes out of the flour mill is sifted and finally mixed, by hand, with shimmering globs of mercury to pull out the gold. If the miner is lucky, a tiny aspirin-sized lump of precious metal will fall out of a bag full of rocks.

Nigeria may be Africa's largest oil producer, but there is no sign of that wealth in these simple settlements. Aside from subsistence farming, several miners say, mining is their only way to earn a living.

Adamou Tsiko, one of the leaders of the miners, says it's very difficult to get money here. About six years ago, the international price of gold started to rise, and Tsiko says the men discovered that they could make money from the surrounding rocks.

The miners, who are dressed in rags and living in mud huts, say they can earn $20 or $30 on a good day. Young kids working at the processing sites say they're paid the equivalent of just $2 a day.

A few foreign companies have made some improvements recently to stem the deaths from the mining.

At first the miners were processing the gold ore inside their homes. Doctors Without Borders persuaded most of them to move their operations to the outskirts of town. And Terra Graphics, an environmental engineering company from Idaho, came in and stripped away the lead-laden topsoil at seven contaminated villages.

In one of the villages, a group of Chinese businessmen has also brought in an ore-crushing machine to tackle the dust problem. It looks like a motorized flour mill, but water runs over the ore as it's pummeled, which keeps the lead-laden dust out of the air.

"They are using water. You'll not see dust. Dust is not coming out," says Abdullah Lokoja, a local businessman who helped the Chinese set up this new wet-processing site.

Most of the mining is still done by hand, but there is less dust in the air at the new mining site.

Gayton says he remains "very concerned" about the situation.

Some contaminated villages are still waiting to be cleaned up. The miners have no legal title to their mines. As a result, they have little desire to invest in cleaner processing equipment. And they could move their toxic operations to another part of the bush at any moment.

Gayton says the scale of the lead contamination here is still unknown and the potential for more damage is huge. "It's turning into a long-term crisis. My nightmare," he says. "And I'll admit it. I'm terrified that because we are treating the children and avoiding the immediate short-term consequence, we might be encouraging this unsafe gold rush."

Particularly if the price of gold remains high, people here are going to try to squeeze value out of the earth, any way they can.

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